1. Field of Invention
This invention relates to methods of patterning resists. This invention also separately relates to structures including the patterned resists.
2. Description of Related Art
Photolithography is used to form patterns in resists applied on substrates. Typically, radiation, typically light, is passed through a patterned mask to transfer the pattern of the mask into the resist. After development, the pattern exists in the resist.
Organic-based resists or photoresist include blends of polymeric and other organic and inorganic materials. The two broad classifications of resists are negative and positive working resists, which produce negative and positive images, respectively. In negative working resists, regions that are exposed to radiation, typically light, are polymerized and, consequently, more insoluble to a developer. Thus, the regions that are not exposed to light are more soluble to the developer and can be preferentially removed relative to the exposed regions during development. In contrast to negative resists, when regions of positive resists are exposed to light, they are chemically altered to exhibit a higher degree of solubility, so they can be preferentially removed relative to the non-exposed regions during development.
Photoresists have been used as structural layers in micro-mechanical devices, as described, for example, in S. Hagen et al., xe2x80x9cPhotosensitive Polyimide: Lithography in the Thick-Film Regime,xe2x80x9d Proceedings 11th International Conference Photopolymers Principles, Processes, and Materials, Society of Plastics Engineers, Inc., Oct. 6-8, 1997; incorporated herein by reference in its entirety.
Resist layers have been formed in ink jet print heads. Ink jet print heads include flow channels for flowing ink and nozzles for discharging ink droplets onto recording media to form images. Ink jet print heads include an energy source that applies energy to the ink to cause the ink droplets to be discharged out of the nozzles and onto the recording medium. Resist layers have been formed as permanent structural layers that define flow channels in ink jet print heads. See, for example, U.S. Pat. No. 6,294,317 to Calistri-Yeh et al.
Openings, or xe2x80x9cfeatures,xe2x80x9d can be formed in resists with various configurations. The openings can be generally round, rectangular or have other like shapes. The openings can also be relatively narrow and long. In such latter types of openings, the openings are defined by the side walls and bottom walls. The sidewalls can have different angular orientations relative to the upper major (planar) surface of the photoresist layers. For example, the sidewalls can be substantially perpendicular to the major surface to form substantially rectangular openings, known as lines or trenches. The sidewalls can alternatively be tapered relative to the major face. Lines and trenches can be either deep or shallow.
Another type of opening or feature formed in resists is an island. Islands are discrete upstanding structures that are generally parallel to each other. Islands have generally elongated shapes. Vias and other through openings can also be formed in resists.
The openings formed in resists can be characterized by their aspect ratio. The definition of the aspect ratio depends on the amount of taper of the sidewalls that define the opening. FIGS. 1 and 2 show two different opening configurations that have aspect ratios defined by respectively different relationships. FIG. 1 shows a photoresist layer 10 having an upper surface 12 and an opening 14 formed in the surface. The opening 14 has a height h and a width w. The height can be less than or equal to the thickness of the resist layer 10. The side walls 16 defining the opening 14 are perpendicular to the upper surface 12. For the opening 14 having such perpendicular side walls 16, the aspect ratio xe2x80x9cAxe2x80x9d can be defined as the ratio of the height xe2x80x9chxe2x80x9d of the opening 14 to the width xe2x80x9cwxe2x80x9d of the opening 14, i.e. A=h/w. Thus, according to this definition, the aspect ratio xe2x80x9cAxe2x80x9d of an opening can be increased by increasing the height xe2x80x9chxe2x80x9d at a constant width xe2x80x9cwxe2x80x9d. It is common for the aspect ratio to be described according to this relationship.
FIG. 2 shows a negative resist layer 20 formed on a substrate 22. A mask 24 is positioned above the resist layer 20. The mask 24 includes openings 25 having a width xe2x80x9cbxe2x80x9d and separated from each other by a distance xe2x80x9caxe2x80x9d. The resist layer 20 includes an upper surface 28, a lower surface 30, and an opening 32 extending vertically between the upper surface 28 and the lower surface 30 and being aligned with the opening xe2x80x9cbxe2x80x9d in the mask 24. The opening 32 is defined by side walls 34, which are tapered relative to the upper surface 28, such that the width of the opening 32 varies from a width bxe2x80x2 at the upper surface 28 to a width bxe2x80x3 at the lower surface 30. The resist layer 20 has a width axe2x80x2 at the upper surface 28, and a width axe2x80x3 at the lower surface 30. For the opening 32 having such tapered side walls 34, the average aspect ratio xe2x80x9cAxe2x80x9d of the opening 32 can be defined as follows: A=2h/(bxe2x80x2+bxe2x80x3). Likewise, the average aspect ratio of the wall between the openings can be defined as A=2h(axe2x80x2+axe2x80x3).
Methods that have been used to pattern resists, such as photoresists, have not been satisfactory. Namely, these methods have not produced satisfactory opening patterns including fine features with relatively higher aspect ratios in certain selected areas of the resist, and grossly patterned areas, with only little or even no detail, in other selected areas of the same resist layer.
Namely, in known methods of forming opening patterns in resists, a lesser amount of exposure to patterning radiation occurs in areas of resists where finer patterns are to be formed, due to proximity effects by the overlying mask. Areas of negative working resists at which finer patterned details are needed receive less exposure than more open areas and, consequently, remain more soluble, so that increased removal occurs during development of the resist. In contrast, more grossly patterned areas on the same resist, for example, areas having little or no detail, receive a greater amount of light exposure than the finer features, so that the exposed more grossly patterned areas become less soluble. Consequently, there is minimal removal of the more grossly patterned areas during development.
FIG. 3 schematically illustrates the relationship between the resist film thickness (resist film thickness=thickness of the resist film remaining after development/resist film thickness before exposure) versus the exposure dose or energy. As shown, the film thickness remaining after development versus the exposure dose increases rapidly at low exposure doses, and flattens out at high exposure doses. Lower exposure doses can be used to form higher aspect ratio features as compared to higher exposure doses. When exposure doses are high, even though fine areas of the resist receive relatively less exposure than coarse features using conventional mask patterns, the final resist film thickness after development is relatively uniform. However, when low exposure doses are used, to achieve resolution of finer features (i.e., features having relatively higher aspect ratios), the thickness of the resist remaining is on the steep slope portion of the curve. The differences in the exposure dose between resist regions including finer features and regions including coarser features, produces significant differences in surface topography in resists. In known patterning methods, significant post-patterning processing has been required to obtain a sufficiently flat surface under these conditions.
For example, as described above, resists have been used to form permanent structural layers that define ink flow channels and the like in ink jet print heads. Problems have occurred in methods of patterning resists that have been utilized in the manufacture of such devices. In a thermal ink jet print head, more finely patterned areas are typically located in the front (nozzle) portion of the print head, while more grossly patterned areas are typically located near the rear portion of the print head. However, in known patterning methods, to achieve the required resolution in the front portion of a thick resist film, the exposure energy at that portion is low, to achieve high-resolution imaging. During each exposure, the rear portion of the resist receives a greater exposure dose because the pattern is more open at the back portion. Due to the difference in exposure dose between the front and rear portions of the resist, the rear portion becomes greater in thickness relative to the front portion. As a result, the height of the rear portion is often much greater than the height of the front portion of the resist layer. In order to achieve a patterned resist having a substantially uniform surface topography, manufacturers have had to perform substantial post-patterning processing, requiring additional steps including chemical mechanical processing (CMP). Although the post-patterning processing improves the surface topography, it also increases manufacturing costs and significantly decreases product throughput.
Known photolithographic processes can only optimize for a regular feature pattern including either only wide features, or only narrow features, present in the same layer. However, these known processes are unable to optimize films that include both areas with finer features and areas with coarser features or no patterns in the same film. For such films, significant topography variations are produced between these different areas.
Thus, there is a need for a process that can achieve broad range of feature detail in the same resist film, with sufficiently uniform surface topography and without the need to also perform significant post-patterning processing.
In addition, some known photolithographic patterning methods have been unable to satisfactorily pattern features with aspect ratios greater than 1:1. Such difficulties have been especially prevalent in applications in which different portions of a resist film require different feature details, such as significantly different configurations and/or aspect ratios of features.
Thus, there is also a need for a process that can pattern features with high aspect ratios and with different aspect ratios in the same film.
This invention provides methods of making photopatterned structures that can satisfy the above-described needs, as well as other needs. Methods according to the invention can form resist films having different feature details, and with more uniform topography. Methods according to the invention can thus at least significantly reduce the need for post-patterning processing to correct for variations in topography.
In addition, exemplary embodiments of the methods according to the invention can form pattern features in resists that have high aspect ratios. The resists can be formed as single films. Moreover, embodiments of the methods according to the invention can provide patterned resists including features having aspect ratios that exceed the maximum aspect ratio typically obtainable using conventional lithographic processes on cured resists.
As described above, in some devices features having significantly different aspect ratios are needed in different portions of the same ink path. Accordingly, a resist film used in the fabrication of such devices should be capable of being patterned to form features, including higher-aspect ratio features, as well as features having a wide range of different aspect ratios, in a single film. Exemplary embodiments of the methods according to the invention can be used to form patterned resist films suitable for use in such devices.
In addition, methods according to the invention can be used to form patterned resist films for suitable for use in various different types of devices, including ink jet print heads, micro electro-mechanical systems (MEMS), and other devices.
This invention provides methods of manufacturing devices including the patterned resists.
This invention also separately provides structures and devices including the patterned resist films.